Foodborne trematodiases

Key facts

At least 56 million people globally suffer from one or more foodborne trematodiases.

People become infected through the consumption of raw fish, crustaceans or vegetables that harbour the parasite larvae.

Foodborne trematodiases are most prevalent in East Asia and South America.

Foodborne trematodiases result in severe liver and lung disease.

Safe and effective medicines are available to prevent and treat foodborne trematodiases.

Foodborne trematodiases are estimated to affect more than 56 million people throughout the world.

They are caused by trematode worms ("flukes"), of which the most common species affecting humans are Clonorchis, Opisthorchis, Fasciola and Paragonimus.

People become infected through the consumption of raw or poorly cooked food: fish, crustaceans and vegetables that harbour the minute larval stages of the parasites (see Table 1).

Transmission

Foodborne trematodiases are zoonoses, i.e. they are naturally transmissible from vertebrate animals to people and vice versa. Direct transmission is however not possible, as the relevant causative parasites become infective only after having completed complex life-cycles that usually involve stages in intermediate, non-human hosts.

The first intermediate host is in all cases a freshwater snail, while the second host differs: in clonorchiasis and opisthorchiasis it is a freshwater fish, in paragonimiasis it is a crustacean, while fascioliasis does not require a second intermediate host. The final host is always a mammal.

People become infected when they ingest the second intermediate host that is infected with larval forms of the parasite. In the case of fascioliasis, people become infected when the larvae are ingested together with the aquatic vegetables to which they are attached (see Table 1 for details).

Table 1. Epidemiological characteristics of foodborne trematodiases

Disease

Infectious agent

Acquired through consumption of

Natural final hosts of the infection

Clonorchiasis

Clonorchis sinensis

Fish

Dogs and other fish-eating carnivores

Opisthorchiasis

Opisthorchis viverrini,O. felineus

Fish

Cats and other fish-eating carnivores

Fascioliasis

Fasciola hepatica, F. gigantica

Aquatic vegetables

Sheep, cattle and other herbivores

Paragonimiasis

Paragonimus spp.

Crustaceans (crabs and crayfish)

Cats, dogs and other crustacean-eating carnivores

Epidemiology

In 2005, more than 56 million people worldwide were infected with foodborne trematodes and over 7000 people died from infection.

Cases of foodborne trematodiases have been reported from over 70 countries worldwide; however East Asia and South America are the most affected areas. In these regions, infections with foodborne trematodes represent a significant public health problem.

Within countries, transmission is often restricted to limited areas and reflects behavioural and ecological patterns, such as people’s food habits, methods of food production and preparation, and the distribution of the intermediate hosts. Information on the epidemiological status of foodborne trematode infections in Africa is largely missing.

The economic impact of foodborne trematodiases is significant, and is mainly linked to losses in the expanding aquaculture industry due to restrictions on exports and reduced consumer demand.

Symptoms

The public health burden attributable to foodborne trematodiases is predominantly due to morbidity rather than mortality.

Early and light infections often pass unnoticed, as they are asymptomatic or only scarcely symptomatic. Conversely, if the worm load is high, general malaise is common and severe pain can occur, especially in the abdominal region, and this occurs most frequently in the case of fascioliasis.

Chronic infections are invariably associated with severe morbidity. Symptoms are mainly organ-specific and reflect the final location of the adult worms in the body.

Inclonorchiasis and opisthorchiasis, the adult worms lodge in the smaller bile ducts of the liver, causing inflammation and fibrosis of the adjacent tissues and eventually cholangiocarcinoma, a severe and fatal form of bile duct cancer. Both C. sinensis and O. viverrini, but not O. felineus, are classified as carcinogenic agents.

Infascioliasis, the adult worms lodge in the larger bile ducts and the gall bladder, where they cause inflammation, fibrosis, blockage, colic pain and jaundice. Liver fibrosis and anaemia are also frequent.

Inparagonimiasis, the final location of the worms is the lung tissue. They cause symptoms that can be confounded with tuberculosis: chronic cough with blood-stained sputum, chest pain, dyspnoea (shortness of breath) and fever. Migration of the worms is possible: cerebral locations are the most severe.

Prevention and control

Control of foodborne trematodiases aims to reduce the risk of infection and at controlling associated morbidity.

Veterinary public health measures and food safety practices are recommended to reduce the risk of infection, while, to control morbidity, WHO recommends improved access to treatment using safe and effective anthelminthic medicines (drugs that expel the worms).

Treatment can be offered through preventive chemotherapy or individual case-management. Preventive chemotherapy involves a population-based approach where everyone in a given region or area is given medicines, irrespective of their infection status. It is recommended in areas where large numbers of individuals are infected.

Individual case-management involves the treatment of people with confirmed or suspected infection (see Table 2): this approach is more appropriate where cases are less clustered and where health facilities are available.

– In districts where the prevalence of infection is 20%, treat all residents every 12 months
– In districts where the prevalence of infection is < 20%, treat all residents every 24 months, or treat only those individuals reporting the habit of eating raw fish, every 12 months

Fascioliasis

Individual case management

Triclabendazole:– 10 mg/kg in single administration (a double dose of 20 mg/kg can be administered in case of treatment failure)

– In sub-districts, villages or communities where cases of paragonimiasis appear to be clustered: treat all residents every 12 months

WHO response

WHO’s work on foodborne trematodiases is part of an integrated approach to the control of neglected tropical diseases, and includes:

development of strategic directions and recommendations;

support for mapping in endemic countries;

support for pilot interventions and control programmes in endemic countries;

support for monitoring and evaluation of implemented activities; and

documentation of the burden of foodborne trematodiases and the impact of implemented interventions.

WHO is working to include foodborne trematodiases in its mainstream preventive chemotherapy strategy and ensure that their worst consequences (cancer of the bile duct and others) are fully prevented.

WHO has also negotiated an agreement with Novartis Pharma AG whereby the company will donate triclabendazole for the treatment of human fascioliasis and paragonimiasis. The medicines are shipped free of charge to ministries of health that apply for them. WHO invites all endemic countries to take advantage of this donation programme.

In 2012, 608 285 individuals living in endemic countries were reported to have received treatment for foodborne trematodiases. In 2013, this figure decreased to 287 590 due to delays in implementation and rescheduling of large-scale treatment interventions.